1. Field of the Invention
This invention relates generally to the field of endodontic instruments and more particularly to reamers or files used in performing root canal procedures. These instruments are used to remove diseased tissue from the canal prior to sealing and filling the canal cavity with a suitable filler material, such as gutta-percha.
2. Description of the Related Art
One of the more technically difficult and delicate procedures in the field of dentistry is root canal therapy. The root canal of a tooth houses the circulatory and neural systems of the tooth. These enter the tooth at the terminus of each of its roots and extend through a narrow, tapered canal system to a pulp chamber adjacent the crown portion of the tooth. If this pulp tissue becomes diseased or injured, it can cause severe pain and trauma to the tooth, sometimes necessitating extraction of the tooth. Root canal therapy involves removing the diseased tissue from the canal and sealing the canal system in its entirety. If successful, root canal therapy can effectively alleviate the pain and trauma associated with the tooth so that it need not be extracted.
To perform a root canal procedure, the endodontist first drills into the tooth to locate the root canal and then uses instruments of small diameter such as reamers and files to remove the decayed, injured or dead tissue from the canal. These are typically elongated instruments which are rotated and/or reciprocated within the root canal. The primary goal is to remove all of the decayed or injured nerve while leaving the integrity of the root canal walls relatively unaffected. Preserving the integrity of the root canal is important in order to allow proper filling of the root canal void in a homogenous three dimensional manner such that leakage or communication between the root canal system and the surrounding and supporting tissues of the tooth is prevented. Once as much of the diseased material as practicable is removed from the root canal, the canal is then sealed closed, typically by reciprocating and/or rotating a condenser instrument in the canal to urge sealing material such as gutta-percha into the canal.
Root canals are not necessarily straight and are often curved or convoluted. Therefore, it is often difficult to clean the canal while preserving its natural shape. Many instruments have a tendency to straighten out the canal or to proceed straight into the root canal wall, altering the natural shape of the canal and sometimes transporting completely through the canal wall. Also, the openings of many root canals are small, particularly in older patients, due to calcified deposits on the root canal inner walls. Thus the files or reamers must be able to withstand the torsional load necessary to penetrate and enlarge the canal opening without breaking the instrument.
To preserve the natural shape of the root canal and to facilitate insertion of the tip of the instrument into small root canal openings, traditional reamers or files have an elongated working portion having a uniform taper such that the diameter of the working portion increases uniformly from a small diameter at the tip of the instrument to a larger diameter at the base of the working portion. The taper rate may vary from instrument to instrument and commonly ranges from about 0.02 mm/mm to about 0.08 mm/mm. Helical flutes defining cutting edges are typically provided along the tapered working portion to promote tissue removal and desired shaping of the root canal and advancement of dental chips and debris up the expanding diameter of the instrument along the helical flutes and away from the tip of the instrument.
One problem with traditional endodontic instruments used for extirpating and filling a root canal is that the torsional limitations of the instrument are often exceeded, resulting in breakage of the instrument in the canal. Breakage of the instrument can occur as a result of inadequate removal of dental chips which are cut from the wall of the root canal. These dental chips often become engaged between the instrument and the root canal wall resulting in increased friction and excessive torque on the instrument. Inadequate chip removal may occur particularly at or near the tip of the instrument where the flutes are relatively shallow. Build up of debris between the flutes and the canal wall can cause damage to the canal walls and/or inadequate or uneven tissue removal and can also lead to failure of the instrument.
Another problem that can occur is transportation or penetration through the canal wall. This can occur when a straight file or reamer is used to prepare a curved canal. The file often will tend to maintain a straight path into the root canal wall instead of following the natural path of the canal. In some extreme cases, the file can actually perforate or penetrate the wall of the root canal causing injury to the supporting tissues of the tooth. One prior attempt to solve this problem was to provide a dental file or reamer having a smooth-walled, non-cutting pilot tip for guiding the file or reamer into the curved root canal. See, for instance, U.S. Pat. No. 4,299,571 to McSpadden, incorporated herein by reference. While the provision of such a smooth-walled pilot tip represented a significant improvement in the art at the time, the design has several significant drawbacks in certain cases.
One drawback is that the pilot tip, being blunt and smooth, has little or no cutting ability. While the blunt tip can fairly easily wedge its way into the soft, fleshy nerve, there is often difficulty encountered in a calcified root canal which has layers of calcified accretion built up along the inner wall of the canal. It is often difficult in these highly calcified root canals to penetrate through the calcified material to a depth sufficient to allow cutting to begin. When using such files having a blunt tip, the file must essentially burnish or grind its way into the calcified material before entering the canal. This generates significant heat and friction as the tip attempts to burnish its way through the hard calcified material. This can cause pain and heating of the tooth which is undesirable. It can also cause increased torsional loads on the file or reamer which can increase the risk of breakage in the canal and decrease the life of the tool.